1. Field of the Invention
The present invention relates to a constant voltage circuit, and, more particularly, to a constant voltage circuit formed in a semiconductor integrated circuit, such as LSI (Large-Scale Integration).
2. Description of the Related Art
LSI includes a variety of circuits, such as for instance various logic circuits. LSI also includes constant voltage circuits for supplying constant voltages to these logic circuits. In accordance with the recent trend to reduce consumed power of LSI, there is a demand for lower power consumption in constant voltage circuit such as for example, intermittent operation of the constant voltage circuits to reduce consumed power.
Conventionally, a bipolar LSI circuit may have various constant voltage circuits incorporated therein to supply constant voltages to logic circuits, etc. One of the constant voltage circuits is a band gap bias circuit shown in FIG. 1. The band gap bias circuit is a circuit which outputs a voltage signal corresponding to the energy difference between the conduction band and the valence band in silicon (Si).
The bias circuit 20 comprises resistors R20 through R27 and NPN type transistors Q11 through Q15. The base of transistor Q15 is connected to a node N11 between resistors R20 and R27, with an output terminal 21 connected between an emitter of the transistor Q15 and a resistor R23. A PNP power saving transistor Q10 is connected between the node N11 and a ground GND. A control signal PS is supplied to the base of the transistor Q10.
When the control signal PS of a high (H) level is inputted to the transistor Q10, this transistor Q10 is turned off. As a result, a current flows into the bias circuit 20 through the resistor R20. The band gap bias circuit then becomes operable and outputs a constant voltage VCS, less source-voltage dependent and less temperature dependent, from the output terminal 21.
When the control signal PS of a low (L) level is inputted to the transistor Q10 if it is not necessary to output the voltage VCS, this transistor Q10 is turned on. A current then flows into the ground through the node N11 and the transistor Q10. consequently, the potential at the node N11 becomes the same as that of the ground GND, and the band gap bias circuit stops functioning, thus reducing the consumed power.
As the band gap bias circuit stops functioning, the consumed power in the bias circuit 20 itself is reduced, but the transistor Q10 is kept on even in this state, causing a slight amount of current to flow through a resistor R20 and the transistor Q10. Accordingly, slight power consumption is experienced even when the band gap bias circuit 20 stops functioning.